Helicopters which have rotor blades above the airframe can vertically ascend and descend, fly forwards and backwards and hover as is known. In such a helicopter, an N/rev vibration is generated by the rotor blades when the number of the rotor blades is N (N being an integer) and vibrations other than the N/rev vibration are further generated due to effects of the wind received by the helicopter and so on. It is desired that the interior comfort of the helicopter is improved; therefore the vibration transmitted to crews is desirably reduced.
Japanese Translation of PCT application No. 2004-511732 discloses a piezoelectric expansion actuator which can suppress vibration in a structure and reduce the structure volume of the piezoelectric actuator while achieving a remarkable improvement in the efficiency of the force introduction of the piezoelectric actuator, which tends to conflict the structure volume reduction. The disclosed distortion actuator is a piezoelectric distortion actuator for reducing vibration in a structure, characterized in that the distortion actuator includes a piezoelectric stack constituted by d33 piezoelectric elements disposed between output elements fixed to the surface of the structure.